Calcium pyrophosphate deposition disease (CPDD) is a common type of arthritis defined by the presence of calcium pyrophosphate (CPP) crystals in articular cartilage. It is particularly common in the US Veteran population as risk factors for CPDD including advanced age and prior joint trauma are highly prevalent in American Veterans. We recently published a study of US veterans using ICD-9 codes to identify 29,000 American Veterans with the disease in a single 5-year period. While CPDD affects elderly patients in a sporadic fashion, it also occurs prematurely in familial patterns. Studies of familial forms of this disease present exciting opportunities to identify novel therapeutic targets for this currently untreatable arthritis. We recently confirmed that a mutation in the stop codon of TNFRSF11B causes early onset CPDD. TNFRSF11B codes for osteoprotegerin (OPG). The OPG mutation causing CPDD (mtOPG) results in an additional 19 amino acids added to the protein's C terminus near the heparin binding domain. OPG is decoy receptor for receptor activator of nuclear factor kappa B ligand (RANKL). RANKL promotes osteoclast formation, and because OPG blocks its function, reduction of OPG activity results in increased osteoclastogenesis. The phenotype of affected patients with this mutation includes exuberant CPP crystal formation, mild osteopenia and severe joint degeneration, a phenotype consistent with a loss of function of OPG. Indeed, preliminary data demonstrate that recombinant mtOPG displays inefficient inhibition of membrane-bound RANKL resulting in excess osteoclastogenesis in vitro. Interestingly, osteoclast conditioned media potently stimulates biomarkers of CPP crystal formation in chondrocytes. The purpose of this work is to investigate the conceptually innovative hypothesis that mtOPG produces CPDD by increasing osteoclastogenesis in subchondral bone. In specific aim 1, using in vitro and in vivo models, we will show that compared to wild type OPG (wtOPG), mtOPG ineffectively binds to the osteoblast cell membrane. resulting in excess un-opposed membrane-bound RANKL and increased osteoclast formation. Overabundant osteoclasts in subchondral bone produce joint damage and release CPP crystal-promoting factors into articular cartilage. In the second aim, structural studies using chromatography, surface plasmon resonance and small angle x-ray scattering, will demonstrate reduced affinity of mtOPG to bind heparan sulfate and decreased dimerization. The scientific premise of this work stems from careful observations of the phenotype of patients with the OPG mutation and is supported by preliminary in vitro functional studies of mtOPG. These studies are the first to identify the OPG/RANKL/RANK pathway in CPDD and to implicate subchondral bone as a target tissue in this disease. The shared clinical features of patients with CCAL1 and those with sporadic CPDD as well as the existence of available drugs which target this pathway make this work particularly exciting.